1. Field of the Invention
The disclosures herein generally relate to a surface emitting laser element and an atomic oscillator.
2. Description of the Related Art
A vertical cavity surface emitting LASER (VCSEL) is a semiconductor laser, which emits light in a direction perpendicular to a substrate surface. The VCSEL has a feature, compared with an end-face emitting type semiconductor laser, of low cost, low power consumption, small size, high performance, and being easy to be integrated two-dimensionally.
The vertical cavity surface emitting laser has a resonator structure that has a resonator region including an active layer, and upper and lower Bragg reflection mirrors provided above and below the resonator region, respectively (See Japanese Published Patent Application No. 2008-53353). The resonator region has a predetermined optical thickness so that light with wavelength of λ resonates in the resonator region in order to obtain light with an oscillation wavelength of λ. The upper and lower Bragg reflection mirrors are formed by DBRs (Distributed Bragg Reflector) formed by laminating materials having different refraction indices, i.e. a low refraction index material and a high refraction index material, alternately. In the DBR, the low and high refraction index materials are formed so that optical thicknesses are λ/4 taking account of the refraction indices of the respective materials, in order to obtain high reflectance where the wavelength is λ.
Moreover, Japanese Patent No. 2751814, Japanese Published Patent Application No. 2000-58958, Japanese Published Patent Application No. H11-330631, and Japanese Published Patent Application No. 2008-283129 disclose a multi-wavelength surface emitting laser element in which plural surface emitting lasers with different wavelengths are formed in one chip. The multi-wavelength surface emitting laser element is expected to be widely used for multi-wavelength communication and the like. As a method of obtaining such a multi-wavelength surface emitting laser element, Japanese Patent No. 2751814 discloses laminating alternately two kinds of material, which are etched by different kinds of etching liquid, on the oscillator region of the surface emitting laser element to form a wavelength adjustment region, and removing one layer for each of surface emitting lasers with different wavelengths by wet etching, thereby changing a layer thickness of the wavelength adjustment region, and the optical thickness in the resonator region is changed for each surface emitting laser. This method is preferable from a viewpoint of controllability of wavelength of laser light emitted from the surface emitting laser and from a viewpoint of productivity.
Japanese Patent No. 2751814 describes performing wet etching in the wavelength adjustment region of semiconductor for one layer for each of surface emitting lasers with different wavelengths, and forming a semiconductor layer by a recrystal growth or the like, to form a reflection mirror above the wavelength adjustment region. However, in the case of forming a semiconductor layer by the recrystal growth on a surface etched by wet etching, high reliability cannot be obtained.
Moreover, Japanese Patent No. 2751814 or the like describes a structure in which an electric current passes through the wavelength adjustment region formed by the semiconductor layer, and electric resistance increases due to band discontinuities on interfaces between layers of different semiconductor materials in the wavelength adjustment region. Moreover, according to the wet etching for one layer, variation in electric resistance or the like may occur for each of the surface emitting lasers with different wavelengths.
Accordingly, a surface emitting laser element on which plural surface emitting lasers for emitting laser light with different wavelengths are formed and in which electric characteristics or the like of the respective surface emitting lasers are uniform is desired.